It is known, particularly in wireless networks, to encode digital data signals by generating complex-valued numbers connected to a time domain signal, particularly a sinusoid of which the amplitude and phase are determined by the respective complex number. A block of segments can be converted by means of an appropriate transform, usually an inverse ‘Fast Fourier transform’ (FFT) into a set of time domain signals of which the carrier frequencies are chosen to ensure orthogonality between the components of the set. The components are transmitted simultaneously in an OFDM ‘burst’. On reception, the encoding process is reversed, the received signal being sampled (at an appropriate rate) and converted by means of an appropriate transform (e.g. a FFT) into the complex numbers and thence back into the original data signal.
However, a long transmission line, particularly characteristic of those used for communication subsea, imposes significantly more attenuation and phase shift on the higher frequency components of a broadband signal such as an OFDM burst than on the lower frequencies. The present invention is concerned therefore with an improved technique for encoding (and decoding) to reduce the effects of relative attenuation and/or phase shift of the higher frequency components.